Arethusa bulbosa is a northeastern North American orchid species rare in most of its range. This study sought to determine the factors associated with the presence of Arethusa within peatlands and its absence from a priori suitable sites. Twenty-four plots with Arethusa were compared with paired plots from which the species was absent and with 22 plots in peatland where the species has never been present. Vegetation and abiotic variables were recorded within the plots, and anthropogenic variables were identified using aerial photographs. The IndVal method was used to determine whether some species were characteristic of Arethusa habitat, and discriminant function analyses were performed to determine which factors were related to occupied and unoccupied sites. No characteristic species were identified when comparing paired plots. Occupied sites were mainly characterized by lower light availability at ground level (46%; mean), higher Floristic Quality Assessment Index (35), and smaller shrubs (55 cm) than unoccupied paired plots (61%, 32, and 61 cm respectively) from the same peatland. Water pH was the main factor discriminating occupied sites (5.5) from the unoccupied sites in peatlands where the species has never been present (3.9), pH being usually higher in plots with Arethusa. Ten species were characteristic of plots with Arethusa (including Larix laricina, Sphagnum palustre, and Aulacomnium warnstorfii) when compared to a priori suitable sites. Overall, our results suggest that Arethusa settles in poor to moderate Sphagnumdominated fen habitats rather than typical open bog habitats, as usually assumed.
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Vol. 20 • No. 1